Conventionally, a telephone connection to be established is controlled and initiated using a telephone number. Each terminal device or user has a unique serial number, and by dialling this number the user can establish a connection only with this one terminal device or its user. In practice, this technique has been the only possibility for establishing a connection in conventional circuit-switched telephone networks.
The emergence of various data networks has brought along also other ways to control or establish a communications connection between two terminal devices. Today, more and more conventional telephone calls are also transmitted via these networks that were originally intended for data transfer. This provides the possibility of using packet-switched data transfer connections in which the cost of data communications is reasonably low. One example of such networks is the Internet. In the Internet, the contact information of the other party is generally defined or presented in some other way than the conventional telephone number. One possible way of establishing a communications connection in these data networks is to use SIP, the Session Initiation Protocol. The Session Initiation Protocol has been standardized by IETF (Internet Engineering Task Force), and its standard number is RFC 2543. The recipient's or user's address according to SIP is of the form: user@network station[parameters] [additional information], i.e. it resembles the form of e-mail addresses. Thus the SIP protocol makes it possible to use various addresses that resemble e-mail addresses for establishing voice connections as well.
Even today a telephone user can come to a situation where one of the persons to be reached uses a conventional telephone number and some other person to be reached uses a “telephone number” enabled by the SIP protocol, or a connection identifier. In practice, this sets higher requirements for the telephone user, because the contact information of the other party can be any of the ways of presentation described above. In one way or another the telephone user must remain up to date and update the contact information of the other party frequently enough in order to make it as easy as possible to establish a connection. This updating work takes time and is laborious, and therefore it is often neglected.